A magnetoresistive readback (MR) head as described in Robert P. Hunt, "A Magnetoresistive Readout Transducer", IEEE Transactions on Magnetics, Vol. MAG-7, March 1971, pp. 150-154; and David A. Thompson, Lubomyr T. Romankiw, and A. F. Mayadas, "Thin Film Magnetoresistors in Memory, Storage and Related Applications", IEEE Transactions on Magnetics, Vol. MAG-11, July 1975, pp. 1039-1049, detects magnetic fields through the resistance changes of the magnetoresistive material. MR heads are of interest for three main reasons: the voltage output when detecting recorded flux transitions in a magnetic medium is large and proportional to the applied sense current; good linear density resolution can be obtained; and the MR head senses flux (.phi.) as compared to an inductive head which senses the time rate of change of flux, d.phi./dt, making the MR output independent of the relative velocity between sensor and medium. A more complete discussion of the magnetoresistive effect is found in Robert M. White, Introduction to Magnetic Recording, IEEE Press, 1985.
A significant problem that exists with the use of MR heads is Barkhausen noise. Barkhausen noise is caused by the sudden and erratic motion of the domain walls separating different magnetic domains in the magnetoresistive element. Experiments by C. Tsang and S. K. Decker, "Study of Domain Formation in Small Permalloy Magnetoresistive Elements", Journal of Applied Physics, Vol. 53, March, 1983, pp. 2602-2604, indicated that longitudinal demagnetization effects are the main cause of the domain formation. The Barkhausen noise caused by the sudden and erratic motion of the domain walls interferes with the signal being detected by the MR head and can cause erroneous reading of digitally recorded data or additive noise on analog recordings. FIGS. 2-4 are a schematic representation of Barkhausen Noise. In FIG. 2 the MR element 10 is shown in its lowest energy or zero field state with no external field applied. In the zero field state, for instance, the permalloy bar breaks up into two large domains 12, 14 antiparallel to one another with orientation along the direction requiring least energy or the easy direction and two smaller closure domains 16, 18 at the ends of the MR element.
Notice that the MR element 10 in FIGS. 2-4 has a point defect x in the left hand closure domain 16. In FIG. 3 a field has been applied to the MR head normal to its easy axis. This applied field causes the right hand closure domain 18 to increase in area while the left hand closure domain decreases in area. The field also causes the magnetization in the two large domains to rotate. Because the wall cannot smoothly move across the point defect in the left hand closure domain, a buckle results in the wall as shown in FIG. 3. If the field is further increased, the wall will eventually overcome th point defect, but when this occurs the wall "jumps" across the defect resulting in a large instantaneous change in magnetization. This instantaneous change results in a spike in the MR head output which is termed Barkhausen Noise.
Applying a longitudinal bias field to the MR sensor can cause the permalloy bar to orient all of its moment along the applied field direction resulting in a single domain state. The magnitude of longitudinal bias field required is dependent on the aspect ratio (L/H) (where L is length and H is height) of the MR element and the height of the sensor.
Many different methods of producing the required longitudinal bias field have been suggested, see, C. Tsang, "Magnetics of Small MR Sensors), IBM Research Report, 1983. Exchange biasing is capable of producing lognitudinal bias fields large enough to force the sensor to remain in a single domain state even with zero applied field, thus reducing all changes in magnetization to rotation about the easy axis, and eliminating Barkhausen Noise. For example, it has been shown that the erratic domain wall motion responsible for Barkhausen Noise in small magnetoresistive sensors can be suppressed by exchange biasing the permalloy layer with antiferromagnetic FeMn. See, C. Tsang and R. E. Fontana, Jr., "Fabrication and Wafer Testing of Barber-Pole and Exchange-Biased Narrow-Track MR Sensors", IEEE Transacitons on Magnetics, Vol. MAG -18, November 1983, pp. 1149-1151. Tsang, et al. have shown that for MR sensors with dimensions of 10 .mu.m.times.15 .mu.m an effective bias field, or exchange field (H.sub.E), of greater than 15 Oe is required to stabilize the domains. It has also been shown by C. Tsang and Kenneth Lee, "Temperature Dependence of Unidirectional Anisotropy Effects in the Permalloy-FeMn Systems", Journal of Applied Physics, Vol. 53, March 1982, pp. 2605-2607 that as the exchange coupled NiFe-FeMn films are heated, the magnitude of H.sub.E drops linearly to zero at about 150.degree. C. See FIG. 5 which is a comparison of H.sub.E versus temperature for different exchange systems. If a NiFe-FeMn exchange biased head were to heat up due to contact with the media or by some other means, the longitudinal bias field would decrease, resulting in domain motion, noise, and change in sensitivity. This essentially eliminates the effectiveness of such an exchange biased head.
To more fully understand and appreciate the concept of exchange biasing as used above the concept of exchange anisotropy must be understood. Exchange anisotropy is a term that describes an interface phenomenon that occurs between magnetic materials. Exchange anisotropy typically occurs in ferromagnetic-antiferromagnetic systems, but has also been seen in ferromagnetic-ferrimagnetic-antiferromagnetic systems.
In a ferromagnetic material, the spins of adjacent layers of atoms experience an exchange interaction which causes them to align parallel to one another. In an antiferromagnetic material however, the spins of adjacent atomic layers experience an exchange interaction which causes the spins of adjacent layers to align antiparallel. Since the magnetic moment of all lattice planes in the antiferromagnet are equal, the antiparallel arrangement of spins results in a material which has no net magnetic moment. FIGS. 6 and 7 schematically represent the direction of the spins of adjacent planes of atoms for ferromagnetic and antiferromagnetic systems.
Anisotropy is a term that describes the preference of a material to be magnetized along particular geometric directions. This preference is due to the fact that the internal energy of the cyrstal differs when the spontaneous moments of the atoms are directed along different crystallographic axes. A magnetic material that has one axis of preferred magnetization is said to exhibit uniaxial anisotropy. Deposition of a material that exhibits uniaxial anisotropy in a D.C. magnetic field reduces dispersion of the zero field magnetization and defines the uniaxial anisotropy direction to be along the applied field direction. FIGS. 8 and 9, respectively, show M-H loops along the easy (parallel to anisotropy direction ) and hard perpendicular to anisotropy direction) and hard (perpendicular to anisotropy diredion directions of a material that exhibits uniaxial anisotropy.
If an antiferromagnetic material is deposited on top of a ferromagnetic material in the same applied DC magnetic field that the ferromagnet was deposited, the spins of the atoms of the first layer of the antiferromagnet will align with the applied field and couple directly (align parallel) with the spins of the surface atoms of the ferromagnetic mateiral. The direction in which the spins of the atoms of both the ferromagnet and antiferromagnet couple at the interface is called the unidirectional anisotropy direction. The next plane of deposited antiferromagnetic atoms experiences an exchange interaction with the first plane of antiferromagnetic atoms which aligns its spins antiparallel to the unidirectional anisotropy direction. Subsequent antiferromagnetic layers will alternate directions as mentioned previously.
In order to describe what the M-H loop of such an exchange coupled magnetic film would look like, it is necessary to make a few assumptions. First, it is assumed that the applied magnetic field of a measuring device, such as a M-H loop tracer, is large enough to staturate the ferromagnetic material. Second, it is assumed that the anisotropy of the antiferromagnetic material is large enough that the direction of its spins will not be changed by the field of the measuring device. Finally, in order to easily observe the exchange effect, both the Curie temperature of the ferromagnet and the Neel temperature of the antiferromagnet must be greater than room temperature.
The bilayer exchange coupled film is placed in a M-H loop tracer with its unidirecitonal anisotropy direction aligned with the +H field direction of the loop tracer. FIGS. 10 and 11, respectively, shown the spin arrangement in the bilayer film, along with the resultant M-H loop. In order to obtain the M-H loop shown, the loop tracer first sweeps the H field to some positive value at which the ferromagnetic material is saturated. Saturation in the +H field is indicated by some value +M.sub.S. The loop tracer field is decreased to zero field, then it is swept in the negative direction. As the loop tracer field becomes more negative, the ferromagnet eventually saturates in the minus M direction. Since the antiferromagnetic mateiral remains unchanged during this reversal, and the surface spins of the ferromagnetic material are coupled directly to the antiferromagnet by a strong direct exchange interaction, the surface spins of the ferromagnet cannot align with the loop tracer field, causing a twist in the magnetization of the ferromagnet (magnetic domain wall). This induced domain wall results in a high field energy requirement ot saturate the ferromagnet in the -M direction. If the loop tracer field is now swept back towards zero, the energy stored in the induced domain wall will cause the magnetization of the ferromagnet to switch back to the +M direction at a value of H field which is less than that required for saturation in the -M direction. In some cases the magnetization can switch from minus to plus even before the applied field crosses zero. This asymmetrical switching field results in the characteristic shifted hysteresis loop of exchange coupled films. The value of the H field at the effective center of the shifted hysteresis loop is called the exchange field (H.sub.E).
It is now apparent why the direction of the spins of the atoms at the bilayer interface is called the unidirectional anisotropy direction; this is because the bilayer material favors magnetization in this one direction alone.
Essentially, as mentioned above, the materials that are currently available for providing exchange biasing are too temperature dependent. A maerial that is not as temperature dependent and which provides a stronger exchange biasing field than currently used materials is needed.
An additional kproblem with MR head sis the fact that the resistivity as a function of the applied magnetic field is not continuously linear. More exactly, referrring to FIG. 12, the change in resistivity .DELTA..rho./.rho. as a function of H is nonlinear and maximally flat about zero field. To obtain maximum sensitivity while maintaining good linearity it is desirable to shift the curve depicted in FIG. 12 so the steepest and most linear portion of the curve, A, is shifted to the zero-field point. Since the curve is not very steep at the zero-field point of the unshifted curve, only a large change in applied field will result in a detectable difference in response. One technique used to linearize the MR response is the use of a barber-pole which provides a transverse field to the MR head, causing the curve to shift so point A is at or near zero external field. See, C. Tsang and R. E. Fontana, Jr., "Fabrication and Wafer Testing of Barber-Pole and Exchange-Biased Narrow-Track MR Sensors", IEEE Transactions on Magnetics, Vol. MAG-18, No. 6, November, 1982. Ideally, what is desired is a single method that provides a strong enough biasing field in both longitudinal and transverse directions of the MR head to overcome Barkhausen noise and shift the sensitivity, simultaneously. V. A. Seredkin, G. I. Frolov, and V. Yu. Yakovchuk, Sov. Tech. Phys. Lett., 9 (12), December 1983 have shown that exchange anisotropy exists in a multilayer NiFe-TbFe film structure.